BARITE NODULES IN PERMIAN KARST SEDIMENTS …embedded in an argillaceous red groundmass. Karst...

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Festschrift zum 60. Geburtstag von Helfried Mostler Geol. Paläont. Mitt. Innsbruck, ISSN 0378-6870, Bd. 20, S. 87-105, 1995 BARITE NODULES IN PERMIAN KARST SEDIMENTS OF THE NORTHERN GREYWACKE ZONE NEAR KITZBÜHEL (TYROL, AUSTRIA) Karl Krainer With 7 figures, 2 tables and 4 plates Abstract: Spherical to subspherical barite nodules are a common constituent in Lower Permian karst sediments of the Devonian Spielberg Dolomite of the Northern Greywacke Zone near Kitzbühel (Austria). The barite nodules are up to several centimeters in size and composed of large, lath like, randomly oriented, inclusion-pure barite crystals and a thin rim of authigenic quartz, dolomite and mica. It is assumed that the barite nodules are of early diagenetic origin and that originally they were composed of fine- grained barite intergrown with interstitial clay minerals. The unusual mineralogical composition and texture of the nodules were caused by a late diagenetic and particularly an early Alpine very low grade metamorphic overprint. The source of the Ba is unknown; it was probably remobilized from older rocks. Zusammenfassung: In unterpermisehen Karstsedimenten des devonischen Spielbergdolomites (Nördliche Grauwackenzone) treten in der Nähe von Kitzbühel häufig rundliche Barytkonkretionen auf. Die Barytkonkretionen werden bis zu mehrere Zentime- ter groß und bestehen aus großen, leistenförmigen, nicht orientierten, reinen Barytkristallen und einem dünnen Saum aus Quarz, Dolomit und neu gesproßten Hellglimmern. Die Barytkonkretionen entstanden vermutlich frühdiagenetisch durch Ausfällung aus Ba-reichen Porenwässern und waren ursprünglich wohl aus feinkörnigem Baryt mit tonigen Verunreinigungen zusammengesetzt. Die ungewöhnli- che Struktur und Zusammensetzung der Konkretionen wird auf eine spätdiagenetische Überprägung und vor allem auf eine sehr schwache, frühalpidische Metamorphose zurückgeführt. Die Herkunft des Ba ist nicht bekannt, vermutlich wurde das Ba aus älteren Gesteinen remobilisiert. 1. Introduction Barite nodules of diagenetic origin have been described from sediments of different environ- mental settings. They seem to occur most fre- quently in pelagic sediments, particularly of Paleozoic (Silurian, Devonian and Early Carbo- niferous) age, but are also known from red beds and even from lacustrine deposits and coal seams. Barite nodules from pelagic sediments are described for example by BOGOCH et al. (1977), CLARK (1985), CLARK & MOSIER (1989), HOLD- EN (1977), CARLSON & HOLDEN (1979), NUELLE & SHELTON (1986), LAZNICKA (1976), PEPPER et al. (1985), WETZEL (1970) and ZIMMERMANN & AMSTUTZ (1966). Examples of barite nodules in red beds are reported by VAN EEDEN (1937), HAM & MERRITT (1944), KEYSER (1966) SHEAD (1923) and TARR (1933), for example. HEMING- WAY & VONDRA (1992) reported barite nodules from Lower Cretaceous lacustrine sediments of Wyoming. Concretionary barite also occurs in Eocene brown coal seams of the Geiseltal (HAAGE & KRUMBIEGEL, 1968). Barite nodules are also known from modern ocean floor sedi- ments (REVELLE & EMERY, 1951). In this paper the term "nodule" is used as a general term which includes concretions as well as rosettes, which have radiating structures. To the author's knowledge, barite has not yet been reported from (paleo-) karst systems. The barite occurrences described in this study are 87

Transcript of BARITE NODULES IN PERMIAN KARST SEDIMENTS …embedded in an argillaceous red groundmass. Karst...

Page 1: BARITE NODULES IN PERMIAN KARST SEDIMENTS …embedded in an argillaceous red groundmass. Karst breccias formed at the base of cavities and are overlain by fine-grained dolomitic sediments.

Festschrift zum 60. Geburtstag von Helfried MostlerGeol. Paläont. Mitt. Innsbruck, ISSN 0378-6870, Bd. 20, S. 87-105, 1995

BARITE NODULES IN PERMIAN KARST SEDIMENTS OF THE NORTHERNGREYWACKE ZONE NEAR KITZBÜHEL (TYROL, AUSTRIA)

Karl Krainer

With 7 figures, 2 tables and 4 plates

Abstract:Spherical to subspherical barite nodules are a common constituent in Lower Permian karst sediments of the DevonianSpielberg Dolomite of the Northern Greywacke Zone near Kitzbühel (Austria). The barite nodules are up to severalcentimeters in size and composed of large, lath like, randomly oriented, inclusion-pure barite crystals and a thin rim ofauthigenic quartz, dolomite and mica.It is assumed that the barite nodules are of early diagenetic origin and that originally they were composed of fine-grained barite intergrown with interstitial clay minerals. The unusual mineralogical composition and texture of thenodules were caused by a late diagenetic and particularly an early Alpine very low grade metamorphic overprint. Thesource of the Ba is unknown; it was probably remobilized from older rocks.

Zusammenfassung:In unterpermisehen Karstsedimenten des devonischen Spielbergdolomites (Nördliche Grauwackenzone) treten in derNähe von Kitzbühel häufig rundliche Barytkonkretionen auf. Die Barytkonkretionen werden bis zu mehrere Zentime-ter groß und bestehen aus großen, leistenförmigen, nicht orientierten, reinen Barytkristallen und einem dünnen Saumaus Quarz, Dolomit und neu gesproßten Hellglimmern.Die Barytkonkretionen entstanden vermutlich frühdiagenetisch durch Ausfällung aus Ba-reichen Porenwässern undwaren ursprünglich wohl aus feinkörnigem Baryt mit tonigen Verunreinigungen zusammengesetzt. Die ungewöhnli-che Struktur und Zusammensetzung der Konkretionen wird auf eine spätdiagenetische Überprägung und vor allem aufeine sehr schwache, frühalpidische Metamorphose zurückgeführt. Die Herkunft des Ba ist nicht bekannt, vermutlichwurde das Ba aus älteren Gesteinen remobilisiert.

1. Introduction

Barite nodules of diagenetic origin have beendescribed from sediments of different environ-mental settings. They seem to occur most fre-quently in pelagic sediments, particularly ofPaleozoic (Silurian, Devonian and Early Carbo-niferous) age, but are also known from red bedsand even from lacustrine deposits and coalseams. Barite nodules from pelagic sediments aredescribed for example by BOGOCH et al. (1977),CLARK (1985), CLARK & MOSIER (1989), HOLD-

EN (1977), CARLSON & HOLDEN (1979), NUELLE

& SHELTON (1986), LAZNICKA (1976), PEPPER etal. (1985), WETZEL (1970) and ZIMMERMANN &AMSTUTZ (1966). Examples of barite nodules in

red beds are reported by VAN EEDEN (1937),HAM & MERRITT (1944), KEYSER (1966) SHEAD

(1923) and TARR (1933), for example. HEMING-

WAY & VONDRA (1992) reported barite nodulesfrom Lower Cretaceous lacustrine sediments ofWyoming. Concretionary barite also occurs inEocene brown coal seams of the Geiseltal(HAAGE & KRUMBIEGEL, 1968). Barite nodulesare also known from modern ocean floor sedi-ments (REVELLE & EMERY, 1951).

In this paper the term "nodule" is used as ageneral term which includes concretions as wellas rosettes, which have radiating structures.To the author's knowledge, barite has not yetbeen reported from (paleo-) karst systems. Thebarite occurrences described in this study are

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Fig. 1: Location map of the investigated area. Barite mineral-ization (indicated by hatch pattern) occurs at the localitiesWilde Hag and Streiteggalm.

clearly associated with Permian paleokarst fea-tures and therefore appear to represent a new,hitherto unrecognized type of synsedimentary toearly diagenetic barite formation.

The objectives of this study are to give a briefsedimentological description of the host sedi-ments, to characterize the barite nodules in termsof their mineralogical composition and texture,and to discuss the formation of these nodules.

2. Location and Geological Setting

The locality of the Permian karst sedimentscontaining barite concretions is situated in theKitzbühel Alps near the town of Kitzbühel,about 1 km south and southeast of the Kitzbühe-ler Horn (1996 m) (localities "Wilde Hag" and"Streiteggalm"; see fig. 1). The rocks exposed in

this area are part of the Northern GreywackeZone, an Upper Austroalpine tectonic unit, andbelong to the "Tectonic Unit II" according toMAVRIDIS & MOSTLER (1970).

Stratigraphically the Northern GreywackeZone comprises Ordovician to Early SilurianWildschönau Slates (Wildschönauer Schiefer)and intercalated volcanic rocks (porphyroids),Silurian cherts and dark dolomites and dolo-stones of Devonian age (Spielberg Dolomite).The latter is a light grey coloured, massive to in-distinctly bedded bioclastic carbonate sediment,dated as Early to Middle Devonian (Emsian-Ei-felian) (EMMANUILIDIS & MOSTLER, 1970, MA-

VRIDIS & MOSTLER, 1970, MOSTLER, 1970).The Spielberg Dolomite underwent intense

karstification during the Late Carboniferous/EarlyPermian. Karst fissures and cavities are filledwith fine-grained, red karst sediments and barite(figs. 3 and 4). Both, Spielberg Dolomite andkarst sediments, are overlain by a thick sequenceof Permian red beds (Basalbreccia, Spielbachshales, Spielberg conglomerate and Mühlbachshales) summarized as "Prebichlschichten" bySTINGL (1983).

At the locality "Wilde Hag", the Early Per-mian Basalbreccia is 30-40 m thick and com-posed of dolomite clasts derived from the under-lying Spielberg Dolomite, embedded in a shalyto sandy, red-coloured groundmass.

At the locality "Streiteggalm" the Basalbrec-cia and younger sediments have been eroded(see fig. 2).

The rocks of the studied area have experi-enced low-grade Alpine metamorphism. Theformation of stilpnomelane, phengitic musco-vite, chlorite, actinolite and epidote in magmaticrocks, the occurrence of chloritoid, pyrophylliteand mixed-layer paragonite-muscovite in theWildschönau Slates and the lack of prehnite,pumpellyite and lawsonite indicate lower green-schist facies with temperatures of 350-400°C andpressures of 3-8 kb (HOSCHEK et al., 1980, CO-

LINS et al., 1980, SCHRAMM, 1980). "Hüte crys-tallinity" (IC) data are consistent with these re-sults. IC values decrease systematically from Sto N, indicating decreasing intensity of meta-

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Red beds

Basalbreccia (Permian)

Karst sediments with baritenodules

Spielberg Dolomite(Devonian)

Karst cavities filledwith barite andkarst sediments

Fig. 2: Generalized section of the karstified Spielberg Dolo-mite (Devonian) and the overlying Lower Permian red beds(basalbreccia, conglomerates, sandstones).

morphism in this direction (SCHRAMM, 1977,1978, 1980). K-Ar and Rb-Sr ages of illite rangefrom 137 to 92 Ma and indicate an Early Creta-ceous metamorphic overprint (KRALIK et al.,1987).

3. Methods of investigation

Compositional and textural parameters of thekarst sediments and barite nodules were investi-gated by thin section microscopy.

Chemical analyses of mineral phases (detritalmicas, micas of the concretions, dolomite) wereperformed using an electron microprobe (ARL-SEMQ). Operating conditions for wavelength-dispersive analysis were 15 kV acceleratingvoltage, 2O(oA sample current and 200|iA emis-sion current.

The Sr content of the barite was measured byX-ray fluorescence analysis. Cathodolumines-cence (CL) microscopy was performed using aTechnosyn cold-cathode luminoscope.

4. Karst sediments and barite nodules

4.1 Sedimentology of the karst sediments

In the Northern Grey wacke Zone Late Paleo-zoic karstification features are widespread andoccur in Devonian dolomites of the western part(Spielberg Dolomite, Schwaz Dolomite) as wellas in the eastern part (MOSTLER, 1984, STINGL,

1983, KRAINER & STINGL, 1986). Lower Carbo-niferous paleokarst has recently been describedfrom Devonian to Lower Carboniferous lime-stones of the Carnic Alps (SCHÖNLAUB et al.,1991).

Two events of karstification can be recog-nized according to MOSTLER (1984):- a Variscan event during the Visean, and- a late- to post-Variscan event during the Late

Carboniferous and Early Permian.Mineralizations of barite, fahlore, magnesite

and siderite are associated with these paleokarstfeatures.

The surface of the Spielberg Dolomite ischaracterized by a marked karst topographywith karst fissures (joints extended by karst dis-solution) and irregular cavities extending morethan 150 m beneath the surface of the SpielbergDolomite.

Karst fissures and cavities are filled with dif-ferent types of sediment and with barite (figs. 3and 4). Red, fine-grained sediments are mostabundant.

The red karst sediments are mostly massiveto indistinctly laminated; locally the sediment iswell-laminated due to sudden changes in grainsize or composition (carbonate-quartz-mica con-tent).

In a few samples erosional surfaces filled bycoarser, quartz-rich sediments, have been ob-served under the microscope.

The karst sediments are moderately to poorlysorted. Detrital grains including quartz grainsare angular. The original rounding of the dolo-mite rhombs is diagenetically overprinted.Micas and elongated quartz grains are common-ly oriented parallel to the bedding plane.

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Fig. 3: Karst cavities within the Devonian Spielberg Dolomiteat the locality Streitegg Alm, filled by red karst sediment andbarite. Pencil for scale.

Fig. 4: Irregular karst cavities from the deeper part of the Per-mian karst system within the Spielberg Dolomite. The cavitiesare almost completely filled by barite. Pocket lamp for scale.

In most samples grain size ranges from0.03-0.07 mm (coarse silt), in coarser laminaefrom 0.1-0.15 mm (fine sand). Some detritalquartz and mica grains may be up to 0.5 mm indiameter (medium sand).

The dolomite content of the red karst sedi-ments in some samples exceeds 80%. In somelayers detrital quartz is the most frequent graintype and occurs in form of monocrystalline, elon-gated fragments. Detrital micas, which are domi-nated by muscovite and degraded bitotites areabundant in some samples. Some micas are bentor broken due to mechanical compaction. Diage-netically formed hematite is dispersed in thegroundmass and concentrated along cleavageplanes of biotite grains. Hematite is also found aseuhedral crystals locally forming rosettes.

In most samples matrix content is high andconsists of carbonate cement and clay (e.g. inquartz- and mica-rich sediments). Some quartzcement (authigenic overgrowths) is also present.

A variety of sediments occurs within paleo-karst dissolution cavities, including carbonates,sandstones, siltstones and shales:

a) Fine-grained, red to brownish dolomite withvariable clay content (pi. 1, fig. 4). The dolo-mite grains (about 0.05 mm in diameter) areanhedral to euhedral, recrystallized and fre-quently zoned with inclusion-rich cores andclear rims. The dolomite rhombs are alsozoned under cathodoluminescence with brightyellowish-red cores and dull rims, suggestingthat the rims contain more Fe than the cores.In some samples the boundary between coreand rim is gradual. Some dolomite rhombsare characterized by dull luminescent cores.The clay-rich groundmass between the dolo-mite rhombs is composed of illite-sericite,and minor hematite. Detrital micas and angu-lar quartz grains may also occur. Some sam-ples from the "Wilde Hag" contain rock frag-ments of red shales, fine-grained sandstones,polycrystralline quartz of metamorphic originand phyllitic grains with diameters up to sev-eral mm. Accessory constituents are tourma-line and zircon.

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b) Red shales composed of illite-sericite and he-matite, with few dolomite rhombs (up to 0.3mm), detrital micas and angular quartz grains(up to 0.15 mm; mostly monocrystallinequartz) and muscovite-sericite (pi. 1, fig. 5;pi. 3, fig. 3). Detrital feldspars are absent.

c) Several mm thick, fine-grained sandstone lay-ers (mostly 0.1-0.2, max. 0.5 mm) containingangular quartz (mostly monocrystallinequartz), muscovite-sericite, some biotite andaccessory grains of brownish tourmaline andzircon (pi. 3, fig. 5).

d) Karst breccias (rare) composed of denslypacked angular clasts of Spielberg Dolomiteembedded in an argillaceous red groundmass.Karst breccias formed at the base of cavitiesand are overlain by fine-grained dolomiticsediments.

e) Thin hematite crusts are locally observedwithin fine-grained karst sediments, indicat-ing periods of non-deposition.

4.2 Barite nodules

The karst sediments are characterized by dif-ferent types of barite mineralization. Karst cav-ities may be completely filled by barite, particu-larly in the deeper parts of the karst system(fig. 4), or barite occurs within the karst sedi-ments as irregular, non-stratabound mineraliza-tions and concretions (fig. 3) (MOSTLER, 1970,MOSTLER et al., 1982).

Individual bodies of barite are up to 2 mthick. The coarse-grained barite of the karst cav-ities has been mined in the last century.

At the locality Streiteggalm barite concretionsare a common constituent of the fine-grained,red-coloured karst sediments (figs. 5, 6, 7). Bar-ite concretions are less common in karst sedi-ments at the locality Wilde Hag.

The occurrence of barite concretions at theselocalities has first been reported by LEITMEIER

(1936) and VOHRYZKA (1968), who describedthem as "Barytkugeln".

The largest barite concretions with diametersup to 20 cm have been found at the Wilde Hag,

* * •

Fig. 5: Small karst cavity filled by red karst sediment, locality Streitegg Alm. A barite nodule composed of white, large, lath likebarite crystals occurs within the karst sediment.

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Fig. 6: A several cm large elongated nodule composed of coar-se, lath-like barite crystals is embedded in fine-grained redkarst sediments (Streitegg Alm).

whereas at the Streiteggalm the size of the con-cretions varies between 0.2 cm and a few centi-meters. The shape of the concretions is sphericalto oval with the larger diameter parallel to thebedding plane. The color of the barite concre-tions is uniformly white.

Three different types of concretions can be dis-tinguished based on the size and mineralogicalcomposition: dolomite concretions (<1 mm),barite-dolomite concretions (1-5 mm) and al-most pure barite concretions (> 5 mm).

a) Dolomite concretions

These concretions generally do not show adistinct separation into core and rim, althoughquartz is more abundant at the margin and micasare concentrated in the center (pi. 2, fig. 2). Theyare composed of authigenic minerals, particular-ly dolomite, and smaller amounts of quartz andmica. Two phases of dolomite have been recog-nized: dolomite I replaced by quartz, and dolo-mite II which is replacing quartz. Toward themargin of the concretions the mica content andthe grain size decrease. In the center of the con-cretions mica crystals display a radial arrange-ment, individual mica crystalls are up to 0.8 mmin length (pi. 3, fig. 4). Quartz is polycrystallinewith different grain sizes, smaller at the margins,with increasing grain size toward the center.Dolomite occurs as euhedral rhombs at the mar-gin and as irregular patches in the center, replac-ing quartz. Micas are intergrown with dolomiteand quartz and replaced them.

In one sample several smaller concretions(0.5-0.8 mm) composed of a dolomitic core anda quartz rim have been observed. Quartz crystalsshow general increase in size from the rim to thecore of the concretion. Quartz replaces dolomite

Fig. 7: Red karst sediment (siltstone) with abundant small barite concretions. Polished slab. Streitegg Alm.

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in this sample. A few dolomite rhombs andsmall mica flakes are also present within thequartz-rich rim.

b) Dolomite-barite concretions

Dolomite-barite concretions are intermediatein size (1-5 mm) and composed of a barite coreand an outer zone of dolomite (pi. 2, fig. 2). Thecore consists of one or a few randomly orientedbladed and lath-shaped barite crystals. Coarsecrystals of mica are locally present as well. Thedolomitic outer zone (mostly dolomite II) con-tains quartz, small micas and hematite, all beingof authigenic origin.

e) Barite concretions (pi. 1, figs. 1-3; pi. 3,fig. 2, and pi. 4, figs. 1-3)

These concretions exceed 5mm in diameterand consist of randomly oriented, inclusion-pure, bladed and lath-shaped barite crystals.These crystals are surrounded by a thin (< 1mm) rim of authigenic quartz, dolomite andmicas. In some samples the barite crystals aresurrounded by a hematite rim (< 3mm).

The barite crystals are lath-shaped, verycoarse-crystalline (>lcm) and free of impurities(pi. 2, fig. 5). Remaining pore space is filled bydolomite (pi. 1, fig. 1). The barite crystals showprominent cleavage and locally undulóse extinc-tion (pi. 1, fig. 3). Quartz and dolomite replacedthe barite crystals along cleavage planes andcrystal boundaries.

Most barite does not show any visible CL,only a few samples showed dull blue CL colours.Dolomite displays red CL colours.

The rim is composed of fine-crystalline authi-genic quartz and some dolomite. The grain sizeof the quartz increases toward the core of theconcretions. At the rim-core boundary the baritecrystals are replaced by quartz and dolomite(pi. 2, figs. 3 and 4).

Within the rim euhedral dolomite rhombs(dolomite II), micas and a few hematite crystalsgrew and replaced quartz.

Small barite relics within the rim are in opti-cal continuity with the large barite crystals of thecore, suggesting that barite was later replacedby quartz, dolomite and micas along the marginof the concretions. From these observations thefollowing reaction series is suggested. Bariteand dolomite I were the first phases, followed byquartz (replaced barite and dolomite I), dolomiteII (dolomite rhombs, replacing quartz) and final-ly micas and hematite crystals formed, replacingall other phases.

These observations also indicate, that thesmall, barite-free concretions originally were alsocomposed of barite, which during a later phasewere replaced by quartz, dolomite, micas and he-matite.

4.3 Mineral chemistry and cathodolumines-cence microscopy

Micas within the concretions show chemicalcompositions typical of muscovite-phengite. Asthese authigenic micas are small and not abun-dant, it was not possible to separate enough ma-terial for X-ray diffractometry. The micas of theconcretions are characterized by significantlylower Na2O- and TiO2 concentrations as com-pared to detrital micas of the karst sediment (seetable 1).

The carbonate of the matrix of the karst sedi-ment is composed of dolomite with FeO con-tents of about 1.5 wt.%. The dolomite rhombs ofthe matrix are lower in FeO (about 0.4 wt.%).The dolomites of the concretions display vari-able FeO contents of up to 2.1 wt.%. The MnOcontent of all dolomite types ranges from0.1-0.3 wt.% (table 2).

In all five investigated samples the Sr contentof the barite was below 1 wt.%.

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Total

x=7

94,41

x=7 x=10 x=7 x=4 x=4

95,32 95,14 96,84 96,25 96,72

x=5

96,89

x=4

SiO2

TiO2

A12O3

Cr2O3

FeO

MnO

MgO

CaO

Na2O

K2O

47,55

0,01

34,02

0,02

0,8

0,01

1,12

0,1

0,18

10,6

47,37

0,02

37,12

<DL

0,53

0,02

1,12

0,03

0,22

8,89

47,61

0,01

36,68

0,01

0,48

0,01

0,95

0,04

0,17

9,18

47,2

0,6

37,15

<DL

1,06

<DL

0,71

0,03

0,73

9,3

46,59

0,53

38,15

0,01

0,72

0,02

0,65

0,04

1,42

8,12

47,38

0,37,93

<DL

0,98

<DL

0,83

0,02

0,17

8,31

48,21

0,08

36,89

<DL

1,44

0,01

1,63

0,03

0,26

8,34

47,46

0,6

37,8

0,04

0,81

0,01

0,58

0,04

0,77

8,36

96,47

Table 1: Chemical composition of micas in karst sediments and concretions (in wt.%):

1-3: authigenic micas in concretions

4-6: detrital micas of the karst sediment

7-8: detrital micas in siltstones near concretions

CaO

MgO

FeO

MnO

1

x=6

30,48

20,63

1,34

0,15

2

x=6

30,21

20,84

1,48

0,12

3

x=6

30,21

20,71

2,09

0,31

4

x=6

31,37

21,15

0,53

0,12

5

x=7

30,81

21,4

0,41

0,11

Total 52,60 52,65 53,32 53,17 52,73

Table 2: Chemical composition of dolomite:

1-2: dolomite of the matrix

3-4: dolomite near concretions 5: dolomite rhombs within concretions (dolomite II)

3.4 Barite nodules of the Tannheim Beds

Barite nodules from the Cretaceous (LateAptian to Albian) Tannheim Beds of the North-ern Calcareous Alps near Vils (Tyrol) have alsobeen studied for comparative purpose. TheTannheim Beds reach a maximum thickness of

80-90 m and consist of grey pelagic marls (seeTOLLMANN, 1976). Near Vils these marls containbarite nodules. These nodules are composed offine-grained, radiating calcite and barite crystalswith minor interstitial clay. The barite concre-tions are typically 5-10 cm in diameter and ofspherical shape. Septarian cracks are present in

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the center, filled by coarse-crystalline, inclu-sion-pure barite crystals and minor calcite (pi. 3,fig. 1). The nodules are similar to the baritenodules described from the Devonian of west-ern Virginia (CLARK & MOSIER, 1989), from theMississippian of Arkansas (ZIMMERMANN &AMSTUTZ, 1966) and from modern ocean-floorsediments (REVELLE& EMERY, 1951).

Although the barite nodules of the TannheimBeds formed in a different environmental settingcompared with the barite nodules of the karstsediments, this example shows that early diage-netic barite nodules are frequently composed offine-grained, radial barite crystals.

4. Discussion

Barite nodules in sediments are a typical diage-netic feature. Most barite nodules described in theliterature are of early diagenetic origin, in mostcases growth already began in unconsolidatedsediments (BOGOCH et al., 1977, CARLSON &HOLDEN, 1977, CLARK, 1985, CLARK & MOSIER,

1989, HEMINGWAY & VONDRA, 1992, NUELLE &

SHELTON, 1986, PEPPER et al., 1985, WETZEL,

1970). In contrast, LAZNTCKA (1976) suggests thatbarite nodules in Devonian (?)/Mississippian car-bonaceous shales and argillaceous dolomiticlimestones in the Mackenzie Mountains, Canada,formed during late diagenesis postdating the for-mation of carbonate concretions, dolomitization,and an early phase of fracturing.

Most of the diagenetically formed barite nod-ules are composed of grey, fine-grained, com-monly radiating barite with some interstitialclay. In some nodules calcite is present in highamounts (BOGOCH et al., 1977).

In the pelagic environment barite formsbelow the sediment - water interface at the redoxboundary between deeper anoxic waters andoverlying oxygenated waters of the dysaerobiczone (CLARK, 1985, CLARK & MOSIER, 1989,PEPPER et al., 1985).

Although barite is common as cement and inconcretionary form in red bed sediments, little is

known about the geochemical conditions of bar-ite nodule formation. The occurrence of barite inred coloured karstsediments indicates, that thebarite nodules grew in an oxygenated environ-ment under conditions different from that in thepelagic environment.

The deformation of laminae in the enclosingshales and siltstones around the nodules indicatesthat the barite nodules grew within unconsoli-dated sediment. The sulphur isotope compositionof four barite samples yielded 834S values rang-ing from +7.5 to +8.8%O (SCHROLL & PAK,1980), suggesting that the sulphate is derivedfrom Permian seawater or the dissolution of ma-rine Permian evaporites. SCHROLL & PAK (1980)pointed out that the sulphur isotopie compositiondoes not allow a genetic interpretation of thebarite formation.

The growth rates of diagenetically formedspherical concretions from flowing and non-flowing pore waters have been calculated byBERNER (1968, 1971, 1980). He pointed out thatconcretions of calcite, siderite and pyrite of afew centimeters in size may form within a veryshort time span of about 12,000 years, and thatflow rates are relatively unimportant on the gro-wth rates.

It is assumed that the barite nodules in thekarst sediments formed by similar growth ratesduring early diagenetic precipitation from Ba-rich porewaters, and that they originally werecomposed of fine-grained barite with radialinternal structures and interstitial clay.

During burial the barite nodules experiencedrecrystallization and a change in mineralogicalcomposition. Recrystallization of dolomite andof the fine-grained barite resulted in larger, ran-domly oriented, lath-shaped barite and blockydolomite crystals. Pétrographie observationsshow that barite and dolomite I were later re-placed by quartz, dolomite II, newly formedmicas (phengite-muscovite) and hematite. Dur-ing early Alpine metamorphism (HOSCHEK et al.1980, COLINS et al. 1980, SCHRAMM, 1980, KRA-

LIK et al. 1987) these minerals formed withinthe small concretions as well as along the rim ofthe larger barite concretions. Replacement of

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barite and dolomite I occurred from the rim to-ward the center of the nodules and along cleav-age plains and crystal boundaries of the largebarite crystals. The unusual structure and com-position of the barite nodules of the karst sedi-ments from the Kitzbiihler Horn compared tothe early diagenetic barite nodules was causedby this late diagenetic and metamorphic over-print.

The source of the Ba remains an unsolvedproblem. Ba-rich hydrothermal fluids in connec-ton with submarine volcanism as well as biogeo-chemical processes can be ruled out. For similarbarite mineralizations hosted in the Lower De-vonian Schwaz Dolomite near Brixlegg FRIM-

MEL (1989) recently suggested an epigenetic ori-gin from hydrothermal Ba-rich solutions derivedfrom older rocks during Variscan metamor-phism. It is suggested that the Ba of the baritemineralizations and nodules near the Kitzbiihe-ler Horn was remobilized from older rocks (e.g.Wildschönauer Schiefer) or derived from theweathering of Ba-rich feldpars.

Acknowledgements

The author wishes to thank Dr. Richard Tessa-dri (Innsbruck) for electron microprobe analysisand Mag. Dr. Christoph Spötl (Reston-Innsbruck)for constructive comments, suggestions and dis-cussion during preparation of the manuscript.

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scale sedimentary features in the Arkansas Barite Manuscript submitted: October 17, 1994

Plate 1

Fig. 1 : Barite nodule composed of coarse, lath-shaped, randomly oriented barite crystals. Pore space between thebarite crystals is filled by dolomite (D). The barite nodule is embedded in silty karst sediments and sur-rounded by a thin rim of fine-grained quartz and dolomite. Crossed niçois, scale bar = 2 mm.

Figs. 2, 3: Barite nodule embedded in silty karst sediments. Scale bar = 1 mm, 2 = plane light, 3 = crossed niçois.

Fig. 4: Karst sediment composed of a dark, fine-grained groundmass and recrystallized, mostly euhedral dolomiterhombs. Plane light, scale bar = 0.3 mm.

Fig. 5: Red siltstone (karst sediment) composed of anhedral to euhedral dolomite grains, detrital micas, angularquartz fragments and a hematite-pigmented, fine-grained groundmass. Plane light, scale bar = 0.4 mm.

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Plate 2

Fig. 1: Small concretion in fine-grained, red-colored karst sediment. The concretion is formed of authigenic micas,quartz and dolomite. Crossed niçois, scale bar 0.5 mm.

Fig. 2: Small concretion composed mainly of dolomite (D) and some authigenic micas (M). The larger concretionsconsist of coarse barite crystals, surrounded and replaced by authigenic quartz, dolomite and micas. Crossedniçois, scale bar 1 mm.

Fig. 3: Thin section photograph of the outer zone of a barite nodule. Along the margin the large barite crystals are re-placed by fine-grained authigenic quartz, dolomite and micas. Dark, fine-grained karst sediment is visible ontop of the photograph. Crossed niçois, scale bar 0.4 mm.

Fig. 4: Detail of a barite nodule composed of large barite crystals which are replaced by fine-grained, authigenicquartz, dolomite and micas along the margin. Crossed niçois, scale bar 1 mm.

Fig. 5: Detail from the central part of a barite nodule, composed of large, randomly oriented, lath-shaped barite crys-tals. Crossed niçois, scale bar 3 mm.

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Plate 3

Fig. 1 : Thin section photograph of a barite nodule (central part) from the Tannheim Beds. The nodule is composed of

fine-grained barite and calcite crystals. Septarian cracks are filled by coarse barite. Crossed niçois, scale bar

2 mm.

Fig. 2: Barite nodule in fine-grained karst sediment. The nodule is formed of coarse barite crystals and a thin rim of

fine-grained authigenic quartz, dolomite and micas. Crossed niçois, scale bar 2 mm.

Fig. 3: Fine-grained, laminated karst sediment containing a few, anhedral to euhedral dolomite rhombs. Plane light,

scale bar 1 mm.

Fig. 4: Concretion composed of radially arranged authigenic micas, dolomite and quartz. Plane light, scale bar

0.4 mm.

Fig. 5: Karst sediment in form of fine grained sandstone composed of angular quartz and detrital micas in a dolomit-

ic and clay-rich, hematite-pigmented groundmass. Plane light, scale bar 0.5 mm.

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Plate 4

Figs. 1, 2: Elongated barite nodule composed of coarse barite crystals which are surrounded by a thin rim of fine-

grained authigenic quartz, dolomite and micas. Fig 1 under plane light, fig. 2 under crossed niçois, scale

bar 1.2 mm.

Fig. 3: Elongated barite nodule formed of large barite crystals, surrounded by fine-grained karst sediment. Cross-

ed niçois, scale bar 3 mm.

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